The objective of this study is to develop and optimize a reverse genetics system (infectious cDNA clone) for the CCHF virus, genus nairovirus, family Bunyaviridae. CCHF is a serious disease with high mortality (up to 60%) and the potential for human-to-human transmission. CCHF virus is classified as a Biosafety Level 4 (BSL-4) agent and is part of the Category A Pathogen List as defined by the Centers for Disease Control and Prevention (CDC). Case management and treatment would benefit from knowledge of the pathogenic mechanism, which is largely unknown. Using reverse genetics we would like to dissect the basic molecular mechanisms of the virus regarding promoter function, genome packaging, protein expression and protein-protein interactions. This will provide us with insight into the pathogenic mechanism of the virus and will help us to design concepts for antiviral treatment and vaccine development. During the next two years we will concentrate on the following aims: Aim #1: Determination of the L segment sequence of CCHF virus (strain IbAr10200) Aim #2: Optimization of CCHF virus-derived minigenome rescue Aim #3: Mutational promoter analysis of CCHF vRNA segments Aim #4: Rescue of recombinant infectious CCHF virus particles The reverse genetics system (infectious clone) will be the key tool for our future studies. The system can be used to study the regulatory elements for viral transcription and replication. It can also be used to determine the functions of the viral proteins for the virus life cycle and to identify the functional domains of these proteins. Both studies will provide us with the necessary knowledge to develop antiviral drugs for treatment of CCHF. The system will also be helpful to study the immune response and the pathogenesis once appropriate animal models have been developed. Thus, the system will also be helpful to design appropriate vaccine candidates for CCHF. Vaccines and antivirals are two important defense strategies against weaponizable pathogens and could effectively be used to prevent the spread of an organism intentionally released in the environment. Thus, the development of the reverse genetics system is a crucial step on the way towards an effective defense system against this Category A listed pathogen.